In medical imaging, it is necessary to print images with a resolution of at least 300 dpi and have 256 distinguishable grey levels, in order to be able to see enough details in the medical image.
In general, three methods exist to achieve a grey level in an image: area modulated printing, density modulated printing or combined area and density modulated printing.
In area modulated printing, grey levels are achieved by printing on certain places of a printing medium, and not printing on others. Ink with an infinite density is used. Light falling in on the medium passes through areas where nothing is printed, and is absorbed completely by the ink on the printed areas. Different methods are possible to print an image in this way: using one pixel with different dot-sizes, using different pixels with one dot-size or using different pixels with different dot-sizes.
U.S. Pat. No. 6,102,513 describes a method and apparatus for printing an output image on a receiver medium in response to an input image file defined by a plurality of pixels. Each pixel obtains a pixel value. The apparatus includes a print head with a plurality of nozzles. Each of the nozzles is capable of ejecting a plurality of ink droplets therefrom. The centres of ink droplets of different volumes are placed at the centre of a pixel on the receiver. In this way, ink spots of different diameters or sizes are symmetrically placed within pixels on the receiver.
It is very difficult to use an area-modulated printing method to obtain 256 grey levels at 300 dpi for a transparent image, because the printed dots need to be very small (smaller than 10 μm) in order to come below the Kanamori curve, which is a curve taking into account the non-linearity of the sensitivity of the human eye. If an area with a first density lies within an area with a second density, and the density difference between both lies under the Kanamori curve, then the area with the first density is indistinguishable from the area with the second density. Furthermore, the dots have to be placed very accurately. If it is desired to have a density of 3 with an infinite density ink, an area of {fraction (1/1000)}th of the pixel should not be covered (white area), as the relationship between density and transparency is given by T=10−D, D being the density and T being the transparency. This means that the droplets have to be placed with an accuracy of       1    1000  of the resolution. This is 2.7 μm for 300 dpi. If the accuracy is less (the distance between two dots is more than 2.7 μm for a 300 dpi image), stripes will become visible.
A second method to obtain a grey image is density modulated printing, in which grey levels are achieved by printing dots of the same size, but with a different ink density. A different ink density is obtained by reducing the transparency of the ink for certain dots. Light passes more or less through the ink, depending on the density or the colour of the ink. Because it is impossible to have 256 heads each printing a different ink, the ink has to be mixed during printing. This can be done either before jetting, such as described e.g. in U.S. Pat. No. 5,606,351 or in U.S. Pat. No. 6,097,406, or after jetting or on the medium, as described in U.S. Pat. No. 5,889,538 and in U.S. Pat. No. 5,625,397.
In U.S. Pat. No. 5,889,538, ink drops of different colours are ejected towards a printing medium and merge with each other thus forming a dot of a desired colour. Each ink drop has an amount of ink in accordance with print data, and is ejected at a timing corresponding to the amount of ink in the drop. A controller is provided for causing the ink pressure chamber of the printing apparatus to eject ink drops therefrom in accordance with the print data. The method described has the disadvantage that the amount of ink ejected, and the timing of ejection must be very precisely controlled.
In U.S. Pat. No. 5,625,397, plural inks of the same dye, having different densities, are used in a dot-on-dot printing format for printing on a receiving medium. That way, a desired optical density level may be readily achieved.
It is a disadvantage of the above methods that only a limited number of grey levels can be obtained.